Role of Carbide Precipitates and Process Parameters on Achieving Grain Boundary Engineered Microstructure in a Ni-Based Superalloy

Thermo-mechanical processing (one-step and iterative) comprising strain (5, 10, and 15 pct cold rolling) and annealing [at 1273 K, 1323 K, and 1373 K (1000 °C, 1050 °C, and 1100 °C) for different times of 30 minutes, 1 and 2 hours] were employed to realize a grain boundary engineered (GBE) microstructure in alloy 617. Among the single-step routes, the process employing 15 pct cold reduction and annealing at 1373 K (1100 °C) for 1 hour was found to be effective in increasing the fraction of Σ3 boundaries; however, it also induced partial recrystallization. The iterative processing employing lower reductions and higher annealing temperatures failed to realize GBE microstructure. The second-phase carbides in this material effectively pin the boundaries thus requiring higher pre-strain to initiate the boundary migration and subsequent multiple twinning events. The iterative processing designed based on the outcomes of the single step route resulted in GBE microstructure by significantly increasing the Σ3 fraction and substantially disrupting the random high-angle grain boundaries connectivity. The newly added Σ3 boundaries in the GBE microstructure predominantly terminated on (111) plane indicating that they have low-energy configuration. The GBE specimen has shown remarkable resistance to intergranular corrosion as compared to the as-received condition.